At least 280,000 hip fractures occur annually in the U.S., at an estimated cost of $9 billion. While over 90% of these are caused by falls, only about 2% of all falls result in hip fracture. Considerable evidence now exists that the most important determinants of hip fracture risk during a fall are the body's impact velocity and configuration (and in particular, whether contact occurs to the hip region). Accordingly, protective responses for reducing impact velocity, and the likelihood for direct impact to the hip, strongly influence fracture risk. Improved understanding of the nature of such responses, and how these are affected by age-related declines in neuromuscular variables, would enhance our ability to develop exercise- based strategies for hip fracture prevention. Based on the results of epidemiological and biomechanical studies, we hypothesize that two protective responses central to safe landing during a fall are (a) absorbing energy in the lower extremity muscles during the descent phase of the fall, and (b) braking the fall with the outstretched hands. We also hypothesize, based on epidemiological evidence, that the efficacy of these responses associate with strength, flexibility, and reaction time. To test these hypotheses, we will address four aims. Aim 1 is to test whether use of the above protective responses influences young females' ability to avoid impact to the hip, and reduce the impact velocity of the body during falls onto a soft gymnasium mat. Aim 2 is to test whether ancillary measures of balance and lower extremity flexibility and reaction time associate with young and elderly subjects' ability to absorb energy in their lower extremity muscles, and reduce impact velocity when descending from standing to sitting. Aim 3 is to test whether balance and upper extremity strength, flexibility, and reaction time associate with young and elderly subjects' ability to quickly contact an impact surface with the outstretched hands, and absorb energy in the upper extremity muscles during simulated falls. Finally, Aim 4 is to develop and validate complementary mathematical models of falling, and use these to determine how specific impairments (or exercise-induced enhancements) in muscle strength, joint flexibility. Finally, Aim 4 is to develop and validate complementary mathematical models of falling, and use these to determine how specific impairments (or exercise-induced enhancements) in muscle strength, joint flexibility, and reaction time affect fall protective responses and fall severity. By identifying the biomechanical and neuromuscular variables which govern safe landing during a fall, these studies should lead to novel and effective interventions for reducing hip fractures in the elderly.